Population Characteristics of Schilbe mystus (Linne, 1758), from two Different Habitats: Asejire and Oyan Lakes Southwestern Nigeria
The population variability of Schilbe mystus in Asejire and Oyan Lakes located in Osun and Ogun states respectively is studied for the first time and it would serve as a reference for future sustainable fisheries management in various environments. Specimens of African butter catfish Schilbe mystus were caught from Asejire and Oyan lakes and ten morphometric and six meristic features were investigated and used to evaluate the degree of variation among the populations. The morphological characters of fresh specimens of Schilbe mystus collected from Oyan and Asejire Lakes showed differences in the meristic and morphometric characteristics of the two populations but these were not statistically significant (p>0.05; t-test) except right gill raker count. The mean values for all body measurement ratio were higher for specimens from the Oyan Lake than Asejire Lake, except for pectoral fin length/standard length and anal fin length/standard length ratios, however, they were not significantly different (p<0.05). The two populations are not racially distinct due to close similarities between meristic and morphometric characters. Differences observed in the right gill raker counts may be attributed to environmentally induced changes rather than by genetic properties.
Received: February 22, 2011;
Accepted: April 16, 2011;
Published: June 07, 2011
S.mystus is the commonest of the four species of family Schilbeidae
found in Nigerian waters (Reed et al., 1967; Holden
and Reed, 1978). Virtually all species which occur over a large geographical
range exhibit measurable variations in various characteristics in the different
habitats. This variation has a genetic and or environmental component which
is a measure of the population differentiation within a species. This variation
especially in the morphometric and meristic characters could indicate the existence
of differences in various characteristics such as rate of growth, survival,
food conversion and dressing percentage (i.e., the amount of flesh available
after the wastes have been removed in fish processing) (Ezenwa,
1978) . And if some of these characteristics are inheritable, it could be
possible to develop improved strains for commercial production through breeding
Anyanwu and Ugwumba (2002), Britto
and Castro (2002), Evans and Jerry (2006), Buitrago-Suarez
and Burr (2007), Suneetha Gunawickrama (2007)
Devaere et al. (2007), Gharibkhani
et al. (2009), Kavan et al. (2009),
Khalili and Amirkolaie (2010), Wiecazek
and Krzykawski (2010) and Wiecazek et al. (2010)
have used meristic and morphometrics characteristics as well as electrophoretic/genetic
studies to determine cohort variations or to separate populations among widely
distributed species. Electrophoresis provides an additional estimate of background
variability within a species. There is presently no report on population variability
of S.mystus within and outside Nigeria. Information on the variability
of meristic and morphometric characters in populations of different origin could
serve as a reference for future sustainable fisheries management in various
environments. Also the equation derived for morphometric relationships of the
fish can be utilized to find out one character from the other.
This study therefore, has studied the morphometric and meristic features of two populations of S. mystus from Asejire Lake in Osun State and Oyan Lake in Ogun State.
MATERIALS AND METHODS
Study areas: Lake Asejire is a man-made lake constructed on River Oshun in 1972. River Oshun is one of the series of West African rivers which do not drain into the Niger system but discharge directly into coastal lagoons and creeks bordering the Atlantic Ocean. The lake is Y-shaped with two unequal arm length of the Y. The location of Asejire and Oyan lakes is shown in Fig. 1.
The catchment area above the dam is 7,800 km2 and the impounded
area is 2,342 ha. The dam has a normal pool elevation (water level) of 150 m
and maximum flood elevation of 152.4 m. The lake has an approximate gross storage
of 7,403 mL (From the data supplied by the Oyo State Water Corporation).
Oyan lake is also an artificially constructed lake on Oyan river and it started
to fill with the closing of the gates of the dam on the 30th of October 1984
and took 2½ months to fill.
||Map of South-Western Nigeria showing drainage system and location
of Asejire and Oyan Lakes
The average annual flow at the dam site is estimated at 1,770 million m3
and the dam provides a normal level reservoir capacity of 270 million m3.
The climate of the Oyan lake is influenced by the NE-SW movement of a zone of surface discontinuity between maritime (Atlantic) air masses and dry continental (Sahara) air masses.
One hundred specimens of S.mystus with size range 10-18cm standard length
each were selected for morphological studies from both lakes in July 2004. This
size range was used because specimens with this standard length were more frequently
encountered and also larger specimens were not examined, since only fish of
similar size may be logically compared with each other when investigating differences
in morphometrics which are otherwise influenced by size (Friedrich,
1972). The specimens were collected from artisanal fishermen that used cast
nets and gill nets of mesh sizes 50-55 mm for catching the fish. Body measurements
and meristic counts were undertaken in the laboratory and divider and vernier
caliper were used for measuring morphometric features.
The morphometric data were measured as described by Fischer
et al. (1998) and shown in Fig. 2. List are Total
Length (TL), Standard Length (SL), Pre-Orbital Length (POL), Eye Diameter (ED),
Post-Orbital Length (POL 2), Body Depth (BD), Length of Pectoral
Fin (PFL), Length of Anal Fin Base (AFBL), Caudal Peduncle Length (CPL) and
Head Length (HL). The meristic variables were counted using the method of Hubbs
and Lagler (1964) and list are counts of dorsal, pectoral anal rays, left
and right gill rakers and vertebrae. Morphometric and meristic characteristics
were determined from the measurements and counts made from the left side of
the fish except for the gill rakers which were counted on both sides. The gill
rakers on the first gill arch (right and left) were washed in tap water, allowed
to dry and counted thereafter. For vertebrae counts, the fish were dried in
an oven at 60°C for 12 h and this allowed the easy removal of the flesh
on either side without damage to the vertebral column. The counts included atlas
||Meristic and morphometric features taken on Schilbe mystus.
Meristic count : 1; Number of dorsal spine, 2; Number of dorsal fin rays
3; Number of pectoral spine 4; Number of pectoral fin rays, 5; Number of
anal rays 6; Number of gill rakers on first gill arch 7; Number of vertebrae
(internal). Morphometric measurement: A; Total lenght B; standard length
C; Preorbital length D; Eye diameter E; Post orbital length F; Body depth
G; Length of pectoral fin H; Lenght of anal fin base I; Caudal peduncle
length J; Head length
Ayoade et al. (2006) determined and reported
the physico-chemical characteristics (surface water temperature, transparency,
dissolved oxygen content and pH) of the two study areas, as well as environmental
conditions (relative humidity, rainfall and atmospheric temperatures).
Statistical analyses: Simple statistics were employed presenting means, standard deviation, t-test, while also regression and correlation coefficients were computed.
Meristic features: The counts and computations of the meristic characters of S. mystus from the two study locations are shown in Table 1. The average number of each character was similar for the two study sites, although there were some slight differences which were not statistically significant (p>0.05) except for right gill raker count (t = 3.92, p>0.05).
Morphometric characters: The standard lengths of S. mystus from Oyan lake ranged from 10-18.7 cm (Mean = 13.49±4.01), while those of Asejire lake ranged from 10.8-18 cm (Mean = 13.58±3.22 cm). The mean values of the Preorbital Length (PL), Body Depth (BD) and Caudal Peduncle Length (CPL) were the same for the two study locations, while the other morphometric features, ED, HL, PFL, AFBL, POL showed differences which were not statistically significant (p>0.05) (Table 1). The body proportions of S. mystus are presented in Table 2. The mean values for all the body measurement ratios were higher for specimens from Oyan lake than Asejire lake except PFL/SL with value of 17.1±1.23 for specimens from Oyan lake and 17.5±1.63 for specimens from Asejire lake. Also, the AFBL/SL for specimens from Oyan lake was 51.2±2.03, while that of Asejire lake was 55.8±4.25. However, the differences were not statistically significant (p>0.05). Also from Table 2, it can be observed that AFBL is long in this species forming more than 50% of the SL in the species in both lakes. The average ratio of body depth in SL (27.2±3.53 and 26.2±3.19) in Oyan and Asejire lake respectively revealed that it is slightly greater than HL in SL (22.8±2.13 and 21.5±1.93) in Oyan and Asejire lakes, respectively.
||Comparison of meristic and morphometric features of Schilbe
mystus from Asejire and Oyan Lake, Nigeria. Results of t-test comparing
some parameters are also shown
|Value with differant letter indicates non-significant and
significant difference respectively (p>0.05), SD: Standard deviation
||Comparison of the ratios for several morphometric characteristics
in Schilbe mystus studied in populations from Oyan and Asejire lakes
|SL: Standard length; SD: Standard deviation
It is of primary importance in conservation of fishery resources to determine
racial constitution of the stock of fish and its also of fundamental importance
to an understanding of species formation (Radcliffe, 1928).
The nature and existence of racial differences varies greatly. Some are obvious,
while others can only be found by careful statistical studies. In other cases,
there are differences in growth rates and different age groups. Morphometric
and meristic studies play a vital role in the identification of fish species
and also use in racial studies.
This study showed that the numbers of vertebrae in S. mystus from the
Oyan and Asejire lakes were not statistically different but showed identical
range in both populations. This is supported by Barbour et
al. (1982) working on Ilisha africana caught off the Nigerian
coast and Ajado and Edokpayi (2003) who studied Clarias
gariepinus from River Niger and Badagry Lagoon, reported identical numbers
of vertebrae and pelvic fin rays. They reported that environmental factors did
not appear to have considerable influence on vertebrae count of some fish.
The higher number of dorsal rays in Oyan Lake and fewer anal rays compared
to Asejire Lake suggests an inverse relationship in both study areas. This apparently
has also been observed in other species and agrees with Ikusemiju
(1975) who also showed a similar inverse relationship in Nigerian lagoons
in Chrysichthys nigrodigitatus.
The statistical difference in the right gill raker count of S. mystus agreed with earlier studies involving fish species from different water bodies.
The differences could be attributed to environmental factors such as temperature
and water quality (different rainfall regimes) that showed some variations between
the two locations as reported by Ayoade et al. (2006).
The minor differences observed in the morphometric measurements of S. mystus
from the two studied localities that were statistically insignificant agreed
with Ajado and Edokpayi (2003) who made similar observations
on the morphometric characteristics of C. gariepinus. The minor differences
observed in morphometric characters of different populations of S.mystus
may be due to environmental or phenotypic influences within the species.
Although meristic and morphometric features are not significantly different, subtle differences were observed in measurements of anal rays, body depth, anal fin base length, post orbital length, and body measurement ratios and this may be due to differences in ecological conditions of the two lakes which have a great bearing on the growth and development of the body.
In conclusion, the two populations of S. mystus studied are not distinctly different judging from the close similarities obtained in meristic and morphometrics characters.
I am grateful to Prof.S.O.Fagade and Prof.A.A.Adebisi for giving useful suggestions during my Ph.D programme. This work is part of my PhD research work.
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